1. Field of the Invention
This invention relates to an optical disc for reproducing information data by irradiation of a laser light. More particularly, it relates to an optical disc which may be employed advantageously for high density recording.
2. Related Art
In an optical disc, such as a digital audio disc (so-called compact disc) or a video disc, an aluminum reflective film is formed on a transparent base plate, on which phase pits have been previously formed in accordance with information signals, and a hard protective film is formed on the reflective film. When the disc surface of such optical disc is irradiated with a readout light, since the light is diffracted at the phase pits and hence the volume of reflected light is reduced significantly, the information signals may be reproduced by detecting the decrease in the volume of the reflected light.
Meanwhile, with the above described optical disc, the resolution in signal reproduction is mostly determined by the wavelength .lambda. of a light source of a reproducing optical system and the numerical aperture of an object lens, such that satisfactory playback signals free of crosstalk may be obtained when the period of formed pits is above the limit of diffraction (.lambda./2NA).
For this reason, for improving the recording density with this type of the optical disc, it is necessary to reduce the wavelength .lambda. of the light source of the play back optical system(semiconductor laser) and to increase the numerical aperture of the object lens.
However, the wavelength of the light source or the numerical aperture of the object lens cannot be improved beyond certain threshold values. That is, under the current state of the art, the recording density can be improved by a factor of four at most by reducing the wavelength of the light source. On the other hand, if the numerical aperture of the lens is to be increased, it becomes difficult to produce a lens which is subject to aberration to a lesser extent. Even granting that such lens has actually been produced, focusing stability tends to be affected by disc vibrations or skew. For this reason, difficulties are felt in significantly improving the recording density of the optical disc.